Yes, you CAN use how much. OR you can say "in what way" does he/she/it frighten you.....which is probably a better way of phrasing this question. Because if you say "How Much" we don't have a scale for "how much".
The question could also be "to what extent" , which could be answered by answering with words like......... Very much, or not much, or this REALLY frightens me!
Hope that is helpful.